Molding of sheet material



Feb. 6, 1940.

W, V. H UTCHINSON HOLDING 0F SHEET MATERIAL Filed 001:. 14, 1936 4 Sheets-Sheet 1 IHVENTOR M I. Hurcuwsolv ATTORNEY Feb. w. v. HuTcHmsoN 2,189,006

HOLDING 0F SHEET MATERIAL File gi Oct. 14, 1936 4 Sheets-Sheet 2 Fig. 2.

ATTORNEY Feb. 6, 1940. w v, HUTCHlNsON 2,189,006

HOLDING 0F SHEET MATERIAL Filed 001:. 14, 1936 4 Sheets-Sheet 5 IHVEHTOR' w. y Huru-I/NJoM ATTORNEY Patented Feb. 6, 1940 UNITED ;STATES MOLDING OF SHEET MATERIAL William Victor Hutchinson, Kingsbury, London,

, I England Application October 14, 1936, Serial No. 105,522 In Great BritainOctober 18, 1935 Claims.

- The object of this invention is to improve upon the molding of sheet materials, such as for example cellulose acetate, which can be softened by heator other suitable treatment, and when bulges.

softened can be distorted to the desired shape.

When molding such material into articles bounded by curves, bends, sinuousities and corners it is customary to mount the sheet material in-plane form within a frame and to shape-it in a mold involving male and female members. Frequently this results in distortion and creases and irregular thickness-inthe finished article, as the mold parts do not adapt themselves to variations in tension on the sheet material due to corners and Also the makingof both male and female members is expensive. The chief object of this invention is to obviate these disadvantages. Another object of this invention-is to facilitate the manufacture of transparent sheets containing bowed, convexedor bulged parts, e. g. as in the production of coupe heads, wind screens, streamlined covers and the like for use on aircraft, so as to obtain various configurations with substantial uniformity of thickness of sheet material without creases, ridges or clouded parts where the distortion or tensionis somewhat pronounced.

' An important feature of the present invention is to ensure that the plastic sheet material-when molded is free from any blemishes and consequently the apparatus used for molding should be particularly adapted for avoiding excessive tension on the plastic sheet material when in the warm, soft condition. For this purpose the mere stretching or pulling of the softened sheet material over the mould, e. g. by tension on springs, somewhat uniformly distributed around the edges of the sheet material, is quite unsatisfactory. The means for supporting the sheet material must adapt itself to the various configurations of molds so as in effect to lay the plastic material smoothly over the mold'in the appropriate directions without pulling one part of the sheet materialexcessively in relation to anotherpart. Arising outof this, the supporting means for the sheetsmaterial should have one or' moreparts, depending upon the shape of the mold, which yieldreadily without stretching the sheet material, so'that the sheet material maintains uniform thickness throughout. It is, of course, known to stretch plastic sheet material over a mold by tension effected by suitable spring or mechanical means, or by a female die member, and it has already been proposed to stretch sheet material such as leather over molds by anchoring the edges of a sheet of leather to bars which are connected by tension springs to an outer frame. Such an arrangement, however, would be'quite unsatisfactory for molding cellulose acetate. or the like plastic sheet material when itis essential to avoid blemishes and creases in the material, 5

and more particularly when the material should be as transparent as possible, e. g. as on aircraft. Because of the foregoing the present invention provides means for supporting plastic sheet ma.-

terial such as cellulose acetate or like sheet mate- 10 rial for use in the molding of such sheet material by heat treatment in which the material is softened and then shaped over a mold, the said means comprising a mold for the molding of the mate rial to the desired shape,-a frame adaptedto 16' support the sheet material and to be'carriedover themold so as to shape the softened sheet material in the frame to the-shape of the mold,gand a relatively movable element in said frame adapted to receive part of the sheet being molded, so that part of the sheet is supportedby the frame and part by said relatively movable element, said latter element being free to move relatively to the plane or level of the frame under the influence of weight applied thereto. 7 l In one example of this invention, assuming that plane sheet of cellulose acetate has to be shaped to form a substantially saddle shaped member having a certain amount of convexity as for example as required for wind screens, nacelle 30, noses and the like for use on aircraft, a male mold member is shaped to forma die or, former and the sheet material after heating is supported vin a frame which is lowered over the mold or former, and the sheet material is so held at its 35 edges in the framethat some. or one of its edges a are translatively displaceable relatively to the frame to an extent somewhat proportioned to the tension on the sheet material due to its distortion. This can be effected by providing an oblong or 40 square frame and attaching the desired edge or edges of the sheet to one or greater proportion of sides of the frame, and the remaining edge or edges to a bar or bars translatively and /or pivotally movable relatively to the frame. The sheet material has bars of wood clamped to opposite faces of it, or single bars nailed'to one-face whenit is inserted in the heating oven and these barscan be clamped in the frame during molding. They also serve to insulate the chorage during molding, means is provided whereby a movement of translation in two dimensions is available for one or more points of anchorage of the sheet material. For example, some articles have to be molded to such exaggerated curves and corners, that a large amount of flow of the material at certain parts in relation to adjacent parts is necessary. This two dimensional movement can be effected by anchoring the desired parts of the edges of the sheet material to blocks which are slidable longitudinally relatively to one or more bars hinged or slidably mounted in the frame. The blocks can have dovetail projections engaging in correspondingly sectioned channels in the appropriate bars. The bar or bars carrying the sliding blocks can each slide relatively to the frame in a direction transverse to its longitudinal direction, e. g. by depending from a sec-. tional bar or block or pin'slidable in a slotted lug projecting inwards from oneside of the frame. Suitable spring means, or weighted cords can yieldingly oppose movement of the pivoted or slidable bar or bars, and the said dovetail projections can have a close sliding fit to obtain a certain amount of friction. Also tension springs can be anchored to the sliding blocks to yieldingly opposesliding of the blocks.

The apparatus according to this invention can be employed successfully with cellulose acetate or like plastic or thermoplastic sheet material ofvarious thicknesses, for example, 2-9, 40, 60 and 80 thousandths of an inch or more.

l of wood, plaster of Paris, or other suitable mate- In order that my invention may be clearly understood and readily carried into effect I have appended hereto four sheets of drawings somewhat diagrammatically illustrating the molding of variously shaped sheet members, and wherein:

Fig. l is a plan view showing the molding of a transparent coupe head suitable for an Avro Anson aeroplane.

Fig. 2 is a section on the line la-la of Fig. 1.

Fig. 3 is a side elevation of Fig. 1 to show the contour of the mold.

Fig. 4 is a plan view showing the molding of a transparent sloping front window part of an aeroplane fuselage.

Fig. 5 shows one of a plurality of hooked rods adapted 'to carry weights and to be suspended from theframes carrying the sheet material dur-'- ing molding.

Fig. 6 is a diagrammatic View showing combined heating and molding apparatus. Fig. 7 is a detail perspective view showing a modification in which the points of support of the sheet material being molded have movements of translation additionally to the movements obtained by reason of the moving parts of the frame.

Fig. 8 is a plan view, and

Fig. 9 is a broken sectional side elevation view rial. The frame carries a sheet of heated plastic material such as cellulose acetate, the boundary edges of which are indicated by the dotted lines 3. The mold shown in Fig. l is suitable for forming a coupe head and as shown in Fig. 2, it is of arcu- 1 ate cross-section, whilst as shown in Fig. 3-it is also of arcuate longitudinal section having a suitable curved nose 4 of somewhat V shape in plan.

In orderto bend the sheet material accurately to the shape of the mold without forming there-' within the frame I a suitable number of relatively 7 adjustable parts arranged to conform approxim'ately with the outline of the predetermined parts of the mold, and to these parts some of the:

edges of the sheet material are attached. For forming the coupe head shown in Fig. 1, I provide bolts or other suitable axes 5b and 6 opposite sides 15*- a V-shaped frame member 5 which pivots on of the frame, these axes 5b and'ii being inclined,

to conform with the angle of movement of the two limbs of the member 5. In the drawings as shown, it is preferred that the pivots 6 shall also have a movement of translation relatively to the frame I, for which purpose it is passed through a longitudinal bar 7 and not through an adjustable side member of the frame i. This bar 1 is, pivoted at one end to a bolt or other suitable member 8, this pivoted end of the bar I being prolonged slightly beyond the pivot t. The bar? is' free to pivot about the bolt 8, and the sheet mate rial is afiixed to thebar l by one edge, the oppo side member of the frame I, the points of anchorage being indicated by a series of crosses-19. The anchorage can be effected by nails or longitudinal bars between which the edges of the sheet manner described, the frame is placed over the mold 2 and weights are attached at the corners a, b, c, and d. The free end 1 of the bar 1 is temporarilytied by string orsecured by a-releasable catch to the adjacent longitudinal frame gmem ber. A hood H (see Fig. 6) is then lowered over,

the mold and frame, this hood being an inverted sheet metal box in which'are located suitable heating means, e. g. Bunsen burners I2. The Weights are attached to the corners a, b, c and d,

by hook shaped rods i3 such as shown inFig. 5, I

and additional hook shaped rodsare attached to the points ,1 and g. The point 9* may betempo: rarily tied or fastened to the adjacent transverse frame member, so that the hooked rods attached to the points 1 and g may. be weighted-but the weights will have no effect until the string or other fastening means is out or released. When the material reaches a predetermined temperature, so that it is sufficiently plastic to be molded, the weighted points ,7 and g-are released, and the sudden tension results in the sheet material being shaped to the outline of the mold without forming creases or irregular thickness parts therein. The sheet material is secured to the upper edges of the frame members; and it is preferred to clamp the edges of the'sheet mate- 1 site parallel edge being affixed to the longitudinal agrsaooei metho'daa'slpreviously described is adopted; That is'to sayythe sheet material is securedat suitable points within: the frame I in suchrmanner' that during'the; preliminary heating the various parts of the frame are relatively immovable. The mold 2:shown.jinaFig. 4 has a transverse logitudinal curvature with two V-sh'aped ends and therefore afpair of V or L shaped frame members I4.

are pivoted at their ends on axes [5. (The apices I6 .of the two frame members M are vtem- .porarily'secured to the end members of the outer ,upper frame I, .and the sheetcmaterial to be .molded is secured atthe parts indicatedby'the crosses H. to. the members 14. The frame l is weighted at its corners as previously; described,

l and =whcn the sheetmaterial is .sufficientlysoft by ;reason of the heat treatment thecorners it -rial"free at a number'of points a substantial of. the jtwoframe members I4 are released, andthese cornersbeing weighted causes the sheet material to assume the configuration of the mold.,, By' leaving the edges of the sheet mate amount of yield is'obtained which prevents tearing-. and distortion within the predetermined shape. 1 Inaddition to. obtaining yield in this manner-the pivot 8 can fit loosely in the bar I,

and also some of the pivots 15 can loosely fit in the frame members I4. I

I of securing the sheet material relatively fixedly to a bar l9 which pivots at one end 20 relatively to the f rame I, I provide sliding blocks 2| having dovetail section projections 22 which slide in correspondingchannels 2,3 in the bar I9. The sheet material is secured at the desired points, to the has blocks 2| and consequently in addition to the yielding movement obtained by the bar I9 during molding the sheet material can bend more freely about a steeply curved mold by reason of the sliding movemento-f the blocks 2|. The sliding blocks 2| can be adopted with many forms of molding frames, e. g. they can be adopted in the V shaped end members and 14 shown in Figs. 1 and 4.

In the arrangement shown in. Figs. 8 and 9 the outer frame I carries on one or more of its sides an inwardly projecting block 2%. having a curved upper face 2la along which is disposed a longitudinal pair of bars 220. between which one edge'of the sheet material is gripped, the

other edges being secured to suitable parts of the frame I or longitudinal movable bars disposed within the frame. The clamping bars 22a" are secured by a bolt 23a which is free to pivot about the head which abuts against the lower face of the block 2011..

For this purpose the block 204; is provided with a slot 24. By adjusting the nut of the bolt 23a, the frictional resistance to movement of. the clamping bars 22a. can be varied. The clamping bars 22a will be weighted by hooked rods at its ends or other suitable means in addition to weighting'the frame I, and by this means the sheet material can be bent about a steeply curved mold, and the freedom of movement of the edges clamped by the bars 22a. can be predetermined to ensure that the sheet material will follow the desired curvature without excessive strain thereon. v

It will be realised that by means of the present invention a plane or substantially plane sheet of material is supported in aframe at parts of 'YBXlEYOfXIfihGIdi-fi a H QBy reason .of';the ',part or parts required to undergo; considerable" deviation being secured to.

its periphery which in the finished article do not lie substantially out ofjthe plane of the material.

At any part where substantial deviation from the plane of-the-material is, to be effected, the materiali-is se'curedsin a movable frameuor clamp-.3-

Themainframe is disposed over a maledie .and

isQloadedso as to .force the material over the:

contour of the die. The sheet of material carapplication of Lhea't',"so thatitheloading 'Oflthe frame causes thematerial to follow the. conthe, clamp or clamps and free to move with 136-.-

rial over the parts Where considerable convexity arises. 1 Thus, where transparent material is em.-.. 'ployed, the material is not subjectedto stresses ried'by the frameis softened,, conveniently by the tending to'impairfthe transparency and in any c ase the strength of the material is substantially maintainedover the whole area of the article.

xlt will be seen-that the invention'invoives the.

use of, a;male die only .This results in a con; siderable saving over themethods involving snaping of the'material; between two mating dies;

Further, in theproduction'of transparent articlesthe; invention avoids the nipping, of the material; between two,; -dies, which tends seriously to 5 afef fectthe transparency of the material.

The; mounting of the-material in a substantially ne frame ec tatesm sa y s f in since the material; can; be subjected tothe action;

of. the heating elements E2 in the hood ii which therebyfunctions as an-oven which distributes The molding operation is of very gentle char-. acter and thisallows the material to be softened only to a slight extent, in fact so slightly that any tendency of the material to take up minor irregularities of the surface of the .die can beavoided.

Where two or more movable clampsare provided for the same part of the'material, these heat substantially uniformly over the plane and provides for uniform softening of, the material.

may be arrangedto operate independently where v the shape of. the articles necessitates it. Thus,

rial in one direction, after which a second clamp is loaded to give an additional drawing of the material in anotherdirection.

The present invention is also applicable to molding sheet material by pressure between die and matrix, because the sheet material can be held in a frame over the matrix and the die, and then lowered. The movable part orparts of the frame will allow the sheet material to flow in steeply curved parts or depressions in the ma trix. Upon depression of the dies, the movable clamp is free to follow the material that is pressed well out of the plane of the frame, so that the tendency of the dies to stretch or tear the material is overcome. Atthe same time, the movable clamp, by holding a length of. relatively unsoftened material undergoing a large amount of flow, allows this portion of the material to be forced smoothly into the shape of the dies.

Between the portion ofth'e margin held by the movable clamp and adjacent portions of the margin secured to the frame, unsupported lengths may be left to allow for the large flow of the material adjacent the movable clamp. The material in these unsupported lengths may stretch or tear, but this is immaterial, since they are I.

4- v 'aisaooe trimmed away .irom the completed article alonz with the rest or the margin; I

What I claim is: lpMeans 'for supporting cellulose acetate or other thermo-plastic material in sheet form "during treatment in which the material is softened by heat and shaped over a mold, comprising an open frame having means thereon to secure thereto part of theperimeter of the sheet material to be molded, a member having means therer on to secure another part of the-perimeter of the sheet material thereto, hingedly connected to and movable freely in relation to the open frame, and Weighted means connected to said member in order to move the part of the sheet material connected to said member over the mold relatively to the part'of the sheet material co-nnectedto the open frame.

"2. Means 'for supporting cellulose acetate or other thermo-plastic material in sheet form during treatment in which the material is softened by heat and shaped over a mold, comprising an open frame having means thereon to: secure thereto part of the perimeter of the sheet mate-' I rial to be molded, a member connected-to said open frame and freely movable relatively thereto, weighted means secured to and a slidable' member carried by said relatively movable memher, said slidable member-being free to slide on said relatively movable member and having means thereon to secure thereto another part of the perimeter of the sheet material.

3. Means for supporting cellulose acetate or other thermo-plasti'c material in sheet form durmg treatment in which the material is softened said member being by heat: and shaped over'a mold, comprisingr'an I open' frame having means thereon-site secure thereto part of the perimeter of the sheet mate-fv rial to bemolded, a'V shaped me'mber pivotally supported atits ends to voppositeasides of the said frame and freetoswing relatively. to theframe and means on said member tolsecure there-1 v to other parts of the perimeter of th e m terial. it v l I I 4. :Means for supporting cellulose acetate .or other thermo plastic material in sheet f0rm'dur-' 1' ing treatment in Which-the material is softened by heat. andrshaped over amold, comprising' zanjf open framehaving means thereon to secure thereto part of the "perimeter of the sheet materialto bemolded, a plurality of bars pivotally support-1 ed in said frame, a plurality of members carriedi by said bars-and slidable relatively thereto and means on saidmembers to secure thereto other j parts of the perimeter of the sheet materialsframe having'means thereon to secure thereto the frame, and a rnernber sli'dable upon said inclined surface and having means thereon to se-' cure theretoanother part of the perimeter of the sheet material.

WILLIAM VICTOR H TcHmsoN. I 

